There seems to be a bug in the touch event handler on iOS devices.
You can't manually set a label for the first word in a sentence. If the span is longer than one word, you can overcome this bug by swiping from the last span token to the beginning, but not the other way around. However, if the span is one word long and happens to be the first word in the sentence, there's no way to label it at all.
The same bug occurs both on iPhone and iPad.
Thanks for the detailed report – this made it easy to find the culprit. It turned out that one place in the touch handler included a check for whether a new token selection had started, but it checked something like !tokenId, which is false for the first token with ID 0 
I've already fixed this and just need to double-check that it now works. Will include the fix with the next release!
Thanks for a quick response, and it's awesome that you've found the fix already! 